Triangular pyramidal cube corner retroreflection elements have excellent retroreflective performance and are suitably used for a retroreflection article such as a traffic sign.
Conventionally, some proposals have been made in relation to a retroreflection article having excellent wide angle performance which has improved entrance angularity, observation angularity, and rotation angularity.
Many proposals are known and various improvements have been considered since a long time ago in relation to improvement of entrance angularities or observation angularities of such cube corner retroreflection sheet and retroreflection article and particularly triangular pyramidal cube corner retroreflection sheet and retroreflection article. Many of these techniques improve the entrance angularity by inclining optic axes of retroreflection elements.
For example, the U.S. Pat. No. 2,310,790 to Jungersen describes mounting of retroreflection elements in various shapes on a thin sheet. Examples of the triangular pyramidal reflective element mentioned in this U.S. Patent include a triangular pyramidal reflective element with its vertex located at a center of its base plane triangle and its optic axis not inclined and having a regular triangular base plane, and a triangular pyramidal reflective element with its vertex not located at a center of its base plane triangle and having an isosceles triangular base plane. In this U.S. Patent, there is a description of efficient reflection of light to an approaching automobile (improvement of the entrance angularity).
Moreover, the size of the triangular pyramidal reflective element is described as a 1/10 inch (2,540 μm) deep or less. Furthermore, FIG. 15 of the U.S. Patent illustrates a pair of triangular pyramidal reflective elements with their optic axes inclined in plus (+) directions (described later) and an inclination angle (θ) of the optic axes is estimated at about 6.5° from a ratio between lengths of a long side and a short side of a base plane isosceles triangle of the illustrated triangular pyramidal reflective element.
The European Patent No. 137,736B1 to Hoopman describes a retroreflection sheet and a retroreflection article where pairs of inclined triangular pyramidal cube corner retroreflection elements having isosceles triangular base planes are arranged on a thin sheet with their base planes on a common plane in a close-packed state and with the elements of each pair turned around 180° with respect to each other. Optic axes of the triangular pyramidal cube corner retroreflection elements described in this patent are inclined in minus (−) directions described in the present specification and an inclination angle is described as being about 7° to 13°.
Furthermore, the U.S. Pat. No. 5,138,488 to Szczech similarly discloses a retroreflection sheet and a retroreflection article where inclined triangular pyramidal cube corner retroreflection elements having isosceles triangular base planes are arranged on a thin sheet with their base planes on a common plane in a close-packed state. In this U.S. Patent, optic axes of the triangular pyramidal reflective elements are inclined toward a side shared by the two triangular pyramidal reflective elements facing each other and making a pair, i.e., in the plus (+) directions (described later) and the Patent specifies that an inclination angle is about 2° to 5° and that a size of the element is 25 μm to 100 μm.
Described in the European Patent No. 548,280B1 corresponding to the above patent are that a distance between a plane which includes the side shared by the two elements making the pair and which is perpendicular to the common plane and a vertex of the element is not equal to a distance between an intersection point of the optic axis of the element and the common plane and the perpendicular plane, that the inclination angle of the optic axis is about 2° to 5°, and that the size of the element is 25 μm to 100 μm.
As described above, in the European Patent No. 548,280B1 to Szczech, the inclination of the optic axis is in a range of about 2° to 5° in both the plus (+) and minus (−) directions. However, the above-described U.S. Patent and the European Patent to Szczech disclose, as embodiments, only the triangular pyramidal reflective elements 87.5 μm in height (h) and having inclination angles of the optic axes of (−)8.2°, (−)9.2°, and (−) 4.3°.
However, with the above-described technique of improving the entrance angularity and the observation angularity, the rotation angularity cannot be improved.
Various proposals have been made with the aim of improving the rotation angularity. In each of the proposals, orientations of retroreflection elements having triangular base planes in various divided zones are combined to improve the rotation angularity.
For example, there are the U.S. Pat. No. 5,022,739, the U.S. Pat. No. 5,132,841, the U.S. Pat. No. 5,175,645 by Bennett, et al., the U.S. Pat. No. 6,036,322 by Nilsen, et al., the U.S. Pat. No. 5,706,132, the U.S. Pat. No. 5,936,770 by Nestegard, et al., the U.S. Pat. No. 5,898,523 by Smith, and the like.
Although these patents are different in shapes of the divided zones of the elements and orientations of the triangular pyramidal cube corner elements, they are the same in their basic techniques where the retroreflection elements are formed by groups of V-shaped parallel grooves in three directions.
Furthermore, as disclosed in the U.S. Pat. No. 6,883,921B2 to Mimura, there are known retroreflection elements including a retroreflection element disclosed in the present invention and having a projection geometry of a base plane similar to an isosceles trapezoid. However, in the retroreflection elements described in this invention, two cube corner retroreflection elements, i.e., a tetrahedral retroreflection element having a quadrilateral (isosceles trapezoidal) projection geometry and a triangular pyramidal retroreflection element having an isosceles triangular projection geometry are formed to always make a pair by means of the groups of V-shaped parallel grooves in three directions. Moreover, the isosceles triangular cube corner retroreflection element has a disadvantage that it is inferior to the quadrilateral cube corner retroreflection element in reflection efficiency.
Also disclosed in the U.S. Pat. No. 6,083,607 to Mimura is a triangular pyramidal cube corner retroreflection element where the V-shaped grooves in three directions and forming the element have different depths. The retroreflection element disclosed in this patent has a triangular base plane.